Genetic engineers have found a way to limit the ability of the mosquito to transmit disease by boosting the insect's immune system.

Researchers from the US and Taiwan have modified the yellow-fever mosquito to make it produce a powerful antibacterial protein called defensin in its body.

The release of the protein, which is triggered by the mosquito taking a blood meal, may make it impossible for the insect to carry a pathogen such as the malaria parasite.

There are actually places in the world now where we have insecticide-resistant mosquitoes and drug-resistant parasites

Professor Anthony James

The incidence of mosquito-borne diseases is increasing at an alarming rate around the world and scientists are looking at a number of novel solutions to the problem.

But commentators say genetically modifying mosquitoes is only a first step; the next, and perhaps more difficult, task will be getting sufficient numbers of the new insects out into the field to supplant infected natural populations.

Vladimir Kokoza and Alexander Raikhel from Michigan State University, and colleagues, constructed a unique two-part gene and spliced it into the DNA of the yellow-fever mosquito (Aedes aegypti)

Less disease - fewer deaths

One component contained the code for defensin which was produced in an organ called the "fat body", which is not unlike the liver and lymph nodes in vertebrates. The second component drove the production of the protein specifically when the insect ingested a blood meal, the usual source from which the mosquito picks up the pathogen - a parasite or virus.

The researchers found high levels of defensin in the bloodstream of genetically altered female mosquitoes that persisted for 20 to 22 days after a single feeding.

Professor Anthony James, of the University of California-Irvine, one of the co-researchers involved in the study, told the BBC: "The general hypothesis of this work is that it should be possible to put genes into mosquito populations that normally transmit and turn them into ones that don't transmit. This, ultimately, would result in less disease and fewer deaths."

But Dr Jo Lines, a senior lecturer at the London School of Hygiene and Tropical Medicine, said this would not be easy.

Apart from the difficulties of breeding up millions of insects for release, he said previous work had shown that such animals were frequently less fit than the populations they were intended to replace.

Insecticide-resistant mosquitoes

He told BBC News Online: "You would end up with insects that are highly inbred and devoid of genetic variability - the modified strain of Aedes aegypti would come from a single egg, and that is bound to affect its competitiveness.

"And, of course, even if you get your mechanism out there, there is always the possibility that the pathogen can evolve to overcome it. It is by no means a permanent solution."

Dr Lines, who has worked to promote the distribution and use of treated nets to fight malaria, said the current emphasis on genetic approaches to the mosquito problem, whilst very worthy, should not distract from "what will work this year, next year and in five year's time - never mind in 20 year's time."

Professor James conceded there were difficulties in pushing genes out into wild insect populations. Asked which strategy would eventually beat mosquito-borne diseases like malaria, he said: "The only winner so far has been the parasite.

"There are actually places in the world now where we have insecticide-resistant mosquitoes and drug-resistant parasites. What this genetic strategy does is put another technique at our disposal for circumstances where other techniques may not work.

"It's not a question of which one is going to win, but a question of: 'In a particular environment, what's the best strategy to use'."

The research is published in the Proceedings of the National Academy of Sciences.